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Hydration – Water, electrolytes and fluid balance

What’s special about electrolytes and hydration? Most people think of hydration in the context of ‘water’. However, any discussion about the physiology of hydration requires reference to ‘electrolytes’ because body fluids are composed primarily of water and electrolytes. These two are intimately connected and work in concert to allow the body in achieving fluid balance (i.e, the amount of fluid lost from the body is equal to the amount of fluid taken in). The maintenance of fluid as well as electrolyte balance in the body is an essential part of life.


Water is the most abundant compound in the body. It accounts for ~70% of total body weight (TBW) in non-obese adults and ~30% of TBW in obese adults. Gender, age, condition Body water content-01of the person, amount of physical exercise, temperature and humidity are factors that influence the amount of water in the body.

Gender accounts for ~60% of TBW in males and ~50% in females, and this difference is mainly because of the increase in body fat in females.1 The more fat in the body, the lower the percentage of water.

Age also influences the amount of body fluids. Newborn infants’ total body mass can be 80% water and this can be higher in premature infants.1, 2 As we age, there is a gradual decrease in the percentage of body water, leading to a gradual reduction in muscle mass and an increase in body fat.1

Water – a vital nutrient

Water has unique properties, which make it uniquely suited as a vital macronutrient. It can act:

Water is an excellent solvent for ionic compounds and for solutes such as glucose and amino acids.3 It is a highly interactive molecule and plays a key role in metabolism of other macronutrients (e.g., proteins, carbohydrates, fats, etc.)

Transport conceptTransport
Water is the medium in which nutrients are transported inside of cells and wastes removed from cells.3, 4 Think of it as the shuttle bus that transports nutrients in and wastes out of cells.

Temp regulateRegulation of body temperature
Water controls body temperature, both in warm and cold environments. It allows efficient loss of heat from the body even when the outdoor temperature is higher than the body temperature.5

Knee-JointLubricant and shock absorber
Water promotes lubrication of joints and acts as a shock absorber during activities such as walking or running.

However, the water in our body is not present as plain water, even when we do drink plain water. It contains electrolytes and other macronutrients (e.g., carbohydrates, amino acids/proteins, vitamins, etc).


An electrolyte is defined as a substance that develops an electrical charge when dissolved in water. Sodium (Na+), potassium (K+), calcium (Ca2+), and magnesium (Mg2+) are examples of electrolytes that develop a positive charge (called cations) in water. On the other hand, chloride (Cl) and bicarbonate (HCO3) develop negative charges in water (called anions).

Movement of fluids and electrolytes

Body fluids (comprised of water, electrolytes, nutrients, etc) exist in two main compartments: intracellular fluid (ICF) and extracellular fluid (ECF), which account for ~2/3 and ~1/3 of all body fluids, respectively.6, 7 The ICF is contained within the body’s more than 100 trillion cells.8 To control fluid passage across cell membranes, our bodies have developed mechanisms to keep each of the electrolytes within specific concentration ranges inside and outside of cells (see figure).7, 9

So…why are the concentration of electrolytes inside and outside the cell different? This is because virtually all cells in the body maintain a ‘nonzero’ electric potential (a negative voltage) between their interior and the exterior. This serves two basic functions:

⎯ First, it allows a cell to function as a battery, providing power to operate a variety of cellular functions.

⎯ Second, it is used for transmitting signals between different parts of electrically excitable cells such as neurons and muscle cells.

Fluid balance

Fluid balance is a coordinated maintenance of both water AND electrolyte balance by sensitive detectors at various sites in the body linked by neural pathways of the brain. The body uses electrolytes to help regulate nerve and muscle function and to maintain acid-base balance. Electrolytes such as sodium help the body maintain fluid balance between the ICF and ECF. This is because the amount of fluid inside or outside of cells depends on the concentration of electrolytes in it. If the electrolyte concentration is high inside the cell (the ICF), fluid moves into the cell. If the electrolyte concentration is low, fluid moves out. To adjust fluid levels, the body can actively move electrolytes in or out of cells.

Fluid gains and losses

Fluid loss and gain are carefully regulated processes. When fluid is lost from the body, the simplest way to replenish it is through liquid intake. Dehydration is the total loss of body fluid, associated with an increase in blood plasma osmolarity.10 Biologically speaking, when dehydration sets in, blood pressure falls triggering the release of renin from the kidneys, promoting the formation of angiotensin II, which stimulates the thirst center of the hypothalamus.

So, drinking water when you’re thirsty is essentially a biologically built-in mechanism that is triggered by dehydration. However, research shows that as we get older, our kidney functions and the sensation of thirst decrease.11, 12

Thirst is a biologically built-in trigger for dehydration…
signal that you should drink your water

A constant supply of water (from beverages, solid foods and normal metabolism of foods13) is needed to replenish the fluids lost through normal physiological activities. The table provides a snapshot of the average amounts, sources and routes by which we gain or lose fluids.

fluid balance-01

In the US, the reference daily intake (RDI) for water is 3.7 litres per day for >18 yr old males, and 2.7 litres per day for >18 yr old females.14 Scientific research does not support the common misconception that everyone should drink two litres (or about eight 8-oz glasses) of water per day.15, 16

The signs and symptoms of fluid deficit9 include:

  • Thirst
  • Reduced urine output and increasing concentration of the urine (darker yellow color) as the body attempts to conserve water
  • Dry mucous membranes, particularly visible in saliva
  • Sunken eyes
  • Headache and confusion
  • Weak pulse
  • Reduced blood pressure
  • Rapid and shallow breathing

Mild deficit relates to a 2% loss of body weight, a moderate deficit relates to a 5% loss and a severe deficit relates to more than 8% loss of body weight.9

Take home message...

Remain hydrated and make sure your intake of electrolytes is Water-01appropriate. This is especially true of the elderly and children who are at higher risk of dehydration. In particular children who play sports in the heat are not always diligent about keeping hydrated during or after the exercise. So, care should be taken by the parent to make sure they keep their kids hydrated at all times.

Happy hydrating,



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  10. Welch, K. Fluid balance. Learning Disability Practice 13, 33-38 (2010).
  11. Hess, T. [Thirst deficiency in old age]. Deutsche medizinische Wochenschrift 112, 1668-1669 (1987).
  12. Kenney, W.L. & Chiu, P. Influence of age on thirst and fluid intake. Medicine and science in sports and exercise 33, 1524-1532 (2001).
  13. Mellanby, K. Metabolic water and desiccation. Nature 150, 21 (1942).
  14. Campbell, S. Dietary Reference Intakes: Water, potassium, sodium, chloride, and sulfate. Clinical Nutrition Insight 30, 1&hyhen (2004).
  15. Negoianu, D. & Goldfarb, S. Just add water. Journal of the American Society of Nephrology 19, 1041-1043 (2008).
  16. Valtin, H. “Drink at least eight glasses of water a day.” Really? Is there scientific evidence for “8× 8”? American Journal of Physiology-Regulatory, Integrative and Comparative Physiology 283, R993-R1004 (2002).
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2 Responses so far.

  1. beejfred@gmail.com' Fred S. says:

    Thanks for the info!

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